This invention relates to systems for identifying random codes, and more particularly to a system for determining the feedback tap position of a shift register generator pseudorandom code modulator.
The advent of the shift register generator, with its capability of generating a noise-like pseudo-random sequence with a unique correlation property, has resulted in a new family of ultra-secure communication systems. These systems rely on the noise-like properties of pseudo-random code modulation to prevent hostile receiver interception and use cross-correlation techniques to give their receivers a high jamming resistance. The pseudo-random code is generally obtained from a linear shift register that is driven by a clock frequency oscillator. In the secure transmitter, information is usually impressed on the pseudo-random code which is used to modulate a stable local oscillator frequency source with the resulting signal being transmitted. In a secure receiver, the received signal is demodulated to video using a stable local oscillator frequency source identical to that contained in the transmitter. The video is cross-correlated with an internally generated pseudo-random code also identical to that generated in the transmitter. The output from the cross-correlator contains the information content being transmitted.
If a hostile receiver is to intercept the secure communication system's message or prepare a suitable jamming copy, it must determine the oscillator frequency, dock frequency, length of the shift register generator and the shift register generator taps. Several methods, using phase lock loop principles, have been used in the past to determine the oscillator frequency such as a method which can be used in phase locking a reference oscillator to the clock frequency. The length of the shift register generator or the number of stages can also be found. Hence to completely solve the intercept and jamming problems, it becomes necessary to determine the feedback taps employed in the shift register generator.
Several study programs have been directed toward the problem of determination of feedback tap locations through analysis of the received pseudo-random code. One program resulted in a technique with the capability of determining tap position with the aid of a computer. However, the size and complexity of the computer would make it impractical for most applications. Another program developed a system called LISA. The technique employed by LISA (linear sequence analyzer) is to switch in all possible feedback tap combinations, in an internal shift register generator, until the code generated is identical to that being received. However, for a 20-stage shift register generator there are twenty-four thousand possible tap combinations that generate maximum sequences. LISA reduces the number of possible combinations by operating only against those codes produced by a four-feedback tap shift register generator.